1. Field of the Invention
The invention relates to a process for producing colorless or light-colored butylglycosides and/or butyloligoglycosides by treatment with base followed by isolation from an aqueous mixture.
2. Discussion of the Background
Butylglycosides, butyloligoglycosides, and their mixtures are interesting polyols that are available from carbohydrates, a replaceable raw material resource. They are gaining increasing importance as intermediates in the production of polyurethanes and surfactants based on long-chained alkylglycosides.
Butyloligoglycosides are generally prepared by heating saccharides, butanol, and an acid. Substantial amounts of byproducts are formed in this process that impart an unattractive color and an unpleasant odor to the glycosides. These byproducts occur especially when using polysaccharides as starting materials, which have to be reacted under vigorous conditions.
Colorless byproducts are also formed that can lead to discolorations in the further use of the glycosides or in purification operations. This applies particularly to the acid catalyzed transacetalization to form alkyloligoglycosides with long-chained alkyl groups.
Various processes are known for improving the color of the alkyloligoglycosides. Thus, European Patent No. 102 558 describes a process by which the color quality of the glycosides is improved by adding alkali metal borates to the reaction batch. The borates are added in an amount at least equivalent to the catalytic sulfuric acid. Starch cannot be used as a starting material in this process, since the butanolysis of starch is not effectively catalyzed by boric acid.
According to U.S. Pat. No. 4,483,979, dyes can be extracted from alkylpolysaccharides with long-chained alkyl groups using polar solvents under anhydrous conditions. Glycosides with 1 to 2 glycose units are coextracted in this process. Since the butyloligoglucosides of industrial interest with 1 to 3 glucose units are readily soluble in acetone and other polar solvents, these products cannot be selectively freed of colored impurities by this process.
In European Patent No. 99 183, alkylglycosides are prepared from polysaccharides containing water and alcohols. The reaction is carried out in the presence of cosolvents such as methanol, ethanol, ethylene glycol, or acetone. The reaction mixture has a high content of unreacted glucose. The reaction product is improved in color, but requires increased distillation cost for purification because of the cosolvents.
European Patent No. 77 167 describes the use of reducing agents such as hypophosphorous acid in the glucose/butanol/acid reaction mixture. With monosaccharides this process leads to glycosides of improved color, but when using starch under vigorous reaction conditions it leads to dark-colored products.
Dark-colored products are also obtained with starch under vigorous reaction conditions when using hydroxypolycarboxylic acids by the method of U.S. Pat. No. 4,465,828.
European Patent No. 132 046 describes a process for preparing alkylglycosides by which the color quality is improved by neutralizing the reaction batch with an alkali metal alkoxide after the reaction. The optically detectable byproducts are reduced by about half by this process.
Color control of alkylglycosides is described in European Patent No. 165 721. The product is bleached in this case with hydrogen peroxide, and the color is then stabilized with a compound that liberates SO.sub.2. However, this process provides products from which dark-colored alkylglycosides are obtained in transacetalization.